CD39 contributes to immune regulation through the hydrolysis of ATP to AMP in the rate limiting step of extracellular adenosine production. CD39 is expressed on the surface of multiple cells of the immune system including T-cells, Natural killer cells, B-cells, dendritic cells and macrophages. In addition, high levels of CD39 expression have been associated with the tumor microenvironment (TME). Research aimed at reducing immunosuppression by CD39 activity has resulted in the study of a small molecule inhibitor, POM1, and multiple anti-CD39 antibodies selected for blocking enzymatic activity or decreasing CD39 expression.